1. Field of the Invention
The present invention generally relates to an optical disk apparatus for recording and/or reproducing information to and/or from an optical disk, and in particular to an optical disk apparatus having an aberration or tilt correction control system for maintaining an orthogonal relationship between an optical axis of a light beam projected by an optical pickup device and an information recording surface of the optical disk.
2. Description of the Prior Art
High speed random access is possible with disc-shaped recording medium, and a high recording density can be achieved by formatting a disk with a narrow data track pitch and pit pitch.
Optical disks such as a DVD, which is typical, have been widely used in recent years as a large capacity recording medium because of a high recording density. Further advances in recording density have also been achieved to further increase storage capacity. Optical disks, however, are typically made from low rigidity materials such as polycarbonate, and even disk deflection or deformation due to warp or bending caused by the dead weight of the disk per se cannot be ignored.
In the case where original data recorded on, for example, an optical disk medium is reproduced by means of an optical pickup device, if an optical axis of an object lens of the pickup device is not perpendicular to the surface of the disk but inclined at some angle with respect to the orthogonal relationship to the surface of the disk, the beam spot projected onto the disk surface is distorted due to the aberration thereof, and there may undesirably cause a distortion in a waveform of the reproduced signal output of the optical pickup device. It is noted here that the inclination error with respect to the orthogonal relationship is referred to as xe2x80x9corthogonal shiftxe2x80x9d or xe2x80x9ctiltxe2x80x9d, hereinafter.
Moreover, due to the aberration distortion caused in the beam spot, the reflection light beams reflected from adjacent recording pits formed on the disk undesirably interfere with each other. Thus, the modulation degree of the reproduction signal is deteriorated, and there may be caused a shift error in a peak time (referred to as xe2x80x9cpeak shiftxe2x80x9d hereinafter) of the reproduction signal, resulting in occurrence of a discrimination error of the information reproduction (RF) signal to be a problem.
As a method of removing distortion components included in the reproduction signal waveform, there has been conventionally used an adaptive equalizer utilizing a finite impulse response filter (referred to as xe2x80x9cFIR filterxe2x80x9d, hereinafter). In this method, an adaptive equalization is recently carried out in a digital data processing system by previously quantizing the reproduced signal using an A/D converter.
Particularly in recent years, however, the data recording density on the recording medium has been remarkably increased and distortion of the reproduction signal due to inter-code interference of the recorded data on the medium is increased, and also a noise influence in a data transmission path can not be ignored because of reduction in amplitude of the reproduction signal. Also, the reproduction signal is sensibly deteriorated by a slight tilt of the disk or a defocus condition in the playback system.
Specifically, in the case of using an optical disk such as DVD-RAM for compatibly recording and reproducing information, such a tilt and defocus condition badly affect both the recording and reproducing operations. Therefore, the aberration correction must be performed with a further higher accuracy. In this situation, as a method of removing the aberration distortion components included in the reproduction signal waveform, a tilt control or aberration correction control method is effective for maintaining an orthogonal relationship between the optical disk surface and the optical axis of the optical pickup device.
A conventional tilt control apparatus is suggested, for example, in the Japanese Laid-Open Patent Publication No. Tokkai-Sho 61-51630, which discloses that the tilt control apparatus detects an error in the orthogonal relationship between a disk surface and an optical axis of a light beam irradiated onto the disk by using tilt photo-sensors, thereby maintaining the orthogonal relationship based on the orthogonal error signal detected by the photo-sensors.
However, this conventional tilt control apparatus has following problems. Specifically, since a pair of photo-sensors are disposed on the right and left sides of the object lens of the optical pickup device, and the apparatus is made large in size and complicated in construction. Moreover, if the characteristics of the photo-sensors are varied in time lapse, an offset is generated to cause an error in the orthogonal relationship. Therefore, a normal orthogonal relationship cannot be maintained even if the orthogonal error is set to zero. Also, a tilt control cannot be performed in a circumferential direction (i.e., track tangential direction) of the disk.
In order to solve these problems, another conventional tilt control system is suggested, for example, in the Japanese Laid-Open Patent Publication No. Tokkai-Hei 5-174406, which discloses that the orthogonal shit error in the circumferential (track tangential) direction included in the reproduction signal is detected and corrected based on the shift in the peak time (i.e., peak shift) of the reproduction signal during the operation of reproducing the information from the disk in a pulse phase modulation (PPM) system.
However, in recent optical disk apparatuses, the information is mainly recorded on the disk in a pulse width modulation (PWM) system in order to obtain a higher recording density. Therefore, there has been a problem that the above conventional tilt control system adapted to the PPM recording system can not be utilized in the data reproduction system adapted to the PWM recording system. Moreover, there has been a problem that a tilt correction control in the circumferential (.e., track tangential) direction of the disk can not be performed during the data recording operation.
The present invention has been developed to solve these problems and has an object to provide an aberration correction control apparatus or tilt control apparatus, which can be utilized in the data reproduction system adapted to the PWM recording system, and also the tilt correction control in the circumferential (.e., track tangential) direction of the disk can be performed during the data recording operation as well as during the data reproducing operation.
Another object of the present invention is to provide an optical disk apparatus using the improvement of the aberration correction control apparatus or tilt control apparatus.
In order to achieve the objects mentioned above, a first aspect of the present invention provides a tilt control apparatus for controlling to minimize an error of an orthogonal shift in an orthogonal relationship between an optical disk surface and an optical axis of a light beam irradiated from an optical pickup onto the optical disk in combination with an adaptive equalizer adaptively renewing a plurality of tap coefficients of a FIR filter in an optical disk apparatus. The tilt control apparatus comprisses: an orthogonal shift detector for detecting the orthogonal shift of the light beam using the tap coefficients of the adaptive equalizer and generating an orthogonal shift signal in accordance with the detected orthogonal shift; an inclination drive unit for varying the inclination of the optical axis of the light beam to correct the orthogonal shift; and a drive control unit for controlling the inclination drive of the inclination drive unit in accordance with the orhogonal shift signal to minimize the orthogonal shift of the light beam.
The orthogonal shift detector detects the orthogonal shift by comparing at least one pair of the tap coefficients symmetrical with respect to a center position in time delay order thereof.
The number of the plurality of the tap coefficients is odd in time delay order, and the drive control unit controls the inclination drive of the inclination drive unit in a manner that at least a pair of the symmetrical tap coefficients are substantially made coincident with each other.
A second aspect of the present invetion provides an optical disk apparatus for recording and reproducing information to and from an optical disk, which comprises: an optical pickup irradiating a light beam onto an optical disk surface for recording and reproducing the information to generate an analogue reproduction signal therefrom; an A/D converter for converting the analogue reproduction signal into a digital form; an adaptive equalizer receiving the digital reproduction signal from the A/D converter and adaptively renewing a plurality of tap coefficients of a FIR filter; and a tilt control apparatus for controlling to minimize an error of an orthogonal shift in an orthogonal relationship between an optical disk surface and an optical axis of a light beam irradiated from the optical pickup onto the optical disk,
wherein the tilt control apparatus comprises: an orthogonal shift detector for detecting the orthogonal shift of the light beam using the tap coefficients of the adaptive equalizer and generating an orthogonal shift signal in accordance with the detected orthogonal shift; an inclination drive unit for varying the inclination of the optical axis of the light beam to correct the orthogonal shift; and a drive control unit for controlling the inclination drive of the inclination drive unit in accordance with the orhogonal shift signal to minimize the orthogonal shift of the light beam.
Thus, in the information recording operation, the orthogonal shift obtained based on a recording track is previously stored in a temporary storage portion and the stored orthogonal shift is used to conduct the tilt control. Thus, the tilt control apparatus can be utilized in the data reproduction system adapted to the PWM recording system, and also the tilt control in the circumferential direction of the disk can be performed during the data recording operation as well as during the data reproducing operation.
A third aspect of the present invention provides an aberration control apparatus for controlling to minimize an aberration contained in a spot of a light beam irradiated from an optical pickup onto the optical disk in combination with an adaptive equalizer adaptively renewing a plurality of tap coefficients of a FIR filter in an optical disk apparatus. The aberration correction control apparatus comprises: an aberration detector for detecting the aberration of the light beam spot using the tap coefficients of the adaptive equalizer and generating an aberration detection signal in accordance with the detected aberration; an aberration correcting unit for correcting the aberration of the light beam spot; and a correction control unit for controlling the aberration correction of the aberration correcting unit in accordance with the aberration detection signal to minimize the aberration of the light beam spot.
The aberration detector detects the aberration by comparing at least one pair of the tap coefficients symmetrical with respect to a center position in time delay order thereof.
The number of the plurality of the tap coefficients is odd in time delay order, and the correction control unit controls the the aberration correction of the aberration correcting unit in a manner that at least a pair of the symmetrical tap coefficients are substantially made coincident with each other.
The aberration correcting unit is segmented into a plurality of liquid crystal tilt correction elements, each of the liquid crystal elements having an independently variable light refractive index, and the aberration correction control is executed by varying the light refractive indexes of the liquid crystal elements individually.
A fourth aspect of the present invention provides an optical disk apparatus for recording and reproducing information to and from an optical disk, which comprises: an optical pickup irradiating a light beam onto an optical disk surface for recording and reproducing the information to generate an analogue reproduction signal therefrom; an A/D converter for converting the analogue reproduction signal into a digital form; an adaptive equalizer receiving the digital reproduction signal from the A/D converter and adaptively renewing a plurality of tap coefficients of a FIR filter; and an aberration control apparatus for controlling to minimize an aberration contained in a spot of a light beam irradiated from the optical pickup onto the optical disk,
wherein the aberration control apparatus comprises: an aberration detector for detecting the aberration of the light beam spot using the tap coefficients of the adaptive equalizer and generating an aberration detection signal in accordance with the detected aberration; an aberration correcting unit for correcting the aberration of the light beam spot; and a correction control unit for controlling the aberration correction of the aberration correcting unit in accordance with the aberration detection signal to minimize the aberration of the light beam spot.